In addition to the hazards of direct electrical shock, the
production electrician faces other hazards as well, including arc flash
and arc blast. In fact, according the National Fire Protection
Association (NFPA), most electrical accidents that require admission to the
hospital are caused by arc flash burns, not because of electrical shock.
In the United States alone, over 2000 people are admitted to
burn centers with severe burns
due to arc flash each year.
An arc flash is when the air around a conductor becomes
ionized and changes from an insulator to a conductor. When that happens, the
live conductor can discharge through the air to another live conductor or to a
grounded conductor.
The surrounding air can erupt in a plasma ball that engulfs
the air and then dissipates in a fraction of a second. The temperature of the
air can reach 19,427°C (35,000°F). If
anyone is unfortunate enough to be in the arc flash zone, they could be
severely burned.
In addition to the danger of the flash, an arcing conductor
can produce an explosive blast with tremendous pressure. In the presence of the
ultrahigh temperatures produced by an arc flash, copper can vaporize and expand
67,000 times, producing a shower of molten metal.
The blast can reach thousands of pounds per square foot and
cause great damage,including ruptured eardrums, collapsed lungs, concussions,
and internal organ ruptures.
The likelihood of an arc flash increases as the voltage
increases: the higher the voltage the more likely it could happen. The size of
the flash also depends on the voltage — the higher the voltage, the larger the
arc flash — as well as the impedance of the circuit feeding it and the
available fault current.
An arc can be initiated by several triggers. Dust,
impurities, and corrosion of insulators can initiate an arc and cause it to
flash over. Water condensation or dripping water can also create a conductive
path for an arc to flash over.
Sometimes arcs are caused by a person accidentally touching
a live part or dropping a tool into live equipment. Conductors can also flash
over if the voltage is high enough and the gap to another conductor or ground
is short enough. And sometimes the insulating material breaks down, allowing an
arc to jump through it or around it.
NFPA 70E – Standard for Electrical Safety in the Workplace
spells out the flash protection boundary within which a person could receive a
second degree burn in the event of an arc flash. For under 300 volts, the flash
protection boundary is 4 feet, although unqualified personnel are not allowed
within 10 feet unless they are escorted by a qualified person.
We can’t predict when an accident like an arc flash will
take place, but there are steps we can take to protect ourselves in the event
of such an accident. When we’re working around live electrical equipment we can
wear protective clothing such as flame-resistant (FR) clothing and V rated gloves
(gloves rated and tested for a particular voltage), and use V-rated tools.
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